Add optional flags to provides endpoints (#448)

## Issue

Since provides endpoints can be non-optional, can the default value of
`optional` is
[false](https://canonical-charmcraft.readthedocs-hosted.com/stable/reference/files/charmcraft-yaml-file/#endpoint-role-endpoint-name-optional),
these endpoints are assumed non-optional.

## Solution

Set `optional` flag on provides endpoints to indicate correct
optionality.

Author: rpbritton

metadata.yaml

@@ -26,15 +26,18 @@ provides:
     interface: mysql_client
   metrics-endpoint:
     interface: prometheus_scrape
+    optional: true
   grafana-dashboard:
     interface: grafana_dashboard
+    optional: true
 requires:
   backend-database:
     interface: mysql_client
     limit: 1
   certificates:
     interface: tls-certificates
     limit: 1
+    optional: true
   logging:
     interface: loki_push_api
     limit: 1